Modern wireless networks and Unmanned Aerial Vehicles (UAVs) both grapple with one core challenge: spectrum scarcity. Yet, as UAVs promise on-demand coverage whether over busy city streets or remote rural areas traditional, centralized spectrum allocation schemes struggle to keep pace. In response, we present UAVSpectrumChain, a fully decentralized marketplace built on Ethereum smart contracts. In our system, ground users (from mobile devices to IoT gateways) register their spectrum needs and lock a small collateral deposit to guarantee honest bidding. Meanwhile, UAV operators specify their available bandwidth and tune their prices according to both revenue targets and a personal “satisfaction” parameter that reflects their own operational priorities. A single smart contract orchestrates four seamless phases Setup, Pricing, Allocation, and Completion automatically computing market-clearing prices, iteratively matching buyers and sellers until equilibrium is reached, and settling payments without human intervention. We implemented a Solidity prototype on the Sepolia testnet and observed consistent convergence within 5–7 rounds, with predictable gas consumption. A minimal web interface powered by MetaMask offers real-time bidding and transparent dashboards, while on-chain event logs provide an immutable audit trail. UAVSpectrumChain demonstrates how blockchain can foster fair, transparent, and fully automated spectrum trading paving the way for the next generation of UAV-enabled communications.